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Process control

Capacity labels cannot rank rapid fixers

Converts Kodak’s pack-in film capacity of 120 rolls per US gallon to approximately 31.7 rolls per litre on the exact US-gallon basis, sets that figure beside Ilford’s published 24 rolls per litre at 1+4 from the July 2010 Rapid Fixer sheet, and shows that the resulting ~1.32 ratio is a difference between label numbers under mismatched documented processes — not evidence that one product is more efficient — because the sheets differ in working-bath composition, dilution framing, temperature and time statements and replenishment figures, and neither document bridges throughput to silver loading.

Published by AnalogFixer ·

Kodak’s Rapid Fixer pack-in sheet publishes a Manual Processing film capacity of 120 rolls of 135-size, 36-exposure film per US gallon (3.8 L), beside fixing times scoped to a fresh bath. Ilford’s Rapid Fixer fact sheet rates an unreplenished film bath at 24 rolls of the same size per litre. Convert the gallon figure into per-litre units and Kodak looks about a third higher. That arithmetic is correct and still cannot rank the products from these labels alone. The two numbers answer different process descriptions, and neither sheet supplies the missing bridge from roll count to silver loading.

This is a document comparison of two dated instruction sheets. It is not a mixing, handling, disposal or reuse guide, and it does not claim either sheet is current.

The unit conversion is not the hard part

Kodak’s film capacity is stated as 120 rolls per 3.8 litres (1 gallon) of fixing bath — “120 135-size, 36-exposure rolls or 120 8×10-inch sheets or an equivalent area in other sizes.” The sheet gives liquid volumes in metric and U.S. units, so the gallon is a US gallon.

An exact US gallon is 3.785411784 litres. Dividing the published 120 rolls by that volume gives 31.70064628 rolls per litre, which rounds to approximately 31.7 rolls per litre. Kodak’s own 3.8 L rounding is a presentation of the same US-gallon pack; the conversion below uses the exact US-gallon basis rather than dividing 120 by 3.8.

Ilford’s July 2010 Rapid Fixer sheet, under capacity without replenishment, gives general-purpose film at 1+4 as 24×135–36 rolls per litre of working-strength fixer.

On a common unit, the published figures are approximately 31.7 rolls per litre and 24 rolls per litre. Their ratio is 31.7 / 24 ≈ 1.32. That ratio is what a superficial reading offers as an efficiency ranking. Everything that follows is why that reading fails.

Diagram showing incompatible capacity label formats that cannot rank rapid fixers.

What each capacity number applies to

The useful comparison is not the ratio. It is a process matrix of what each published capacity is attached to.

Kodak Rapid Fixer pack-in, © Kodak 2015, code 20370811 Ilford Rapid Fixer, July 2010 footer
Published film capacity 120 rolls of 135-36 (or 120 8×10 sheets) per 1 US gal / 3.8 L 24×135–36 rolls per litre of working-strength fixer
Per-litre after exact US-gallon conversion ≈ 31.7 rolls/L 24 rolls/L as published
Working bath as described Film bath prepared from Solution A and Solution B Concentrate diluted 1+4 for all film fixing
Hardener language on that sheet Names Solution A and Solution B; does not call Solution B a hardener “Non-hardening rapid fixer”; “must not be used with fix hardeners”
TEMP_PLACEHOLDER
Replenishment figure on the same sheet 10 mL replenisher per roll (all tank sizes) 45 mL working-strength fixer per 135–36 at 1+4
Silver concentration threshold on that sheet Not published Film bath “can be allowed to rise to 8–10 g/l without serious effect”

Neither sheet is identified here as current. Kodak is identified by the 2015 copyright line and pack-in code 20370811; there is no separate revision footer on that one-page sheet. Ilford is identified by the July 2010 page footer.

Process mismatch is plural, not a single cause

Several documented differences sit between the two capacity labels at once:

  • Working-bath composition. Kodak’s film working bath is prepared from Solution A and Solution B. Ilford’s film path is a single concentrate at 1+4 and is stated as non-hardening. Kodak’s pack-in examined here never labels Solution B as a hardener; a Dektomatic processor note says not to use Solution B in that machine, which does not by itself identify the part’s function.
  • Dilution framing. Ilford’s 24-roll figure is explicitly for working strength at 1+4. Kodak’s 120-roll figure is for the film fixing bath prepared on that sheet’s film chart, not for a 1+4 concentrate statement.
  • Temperature and time framing. Kodak gives a band at 18–24°C with separate times for most professional films and for T-MAX. Ilford gives average minimum times at 20°C for general-purpose film. Those are related conditions, not identical protocols.
  • Replenishment. Both sheets publish replenishment rates (10 mL per roll on Kodak; 45 mL per 135–36 on Ilford at 1+4), so the capacity labels sit beside different maintenance figures even before anyone asks which bath lasts longer in practice.

None of those differences is offered here as the reason the per-litre numbers diverge. The sheets do not isolate a single variable. Treating the 1.32 ratio as proof that Kodak’s product is more efficient invents a controlled comparison the documents do not contain. It is a difference between label numbers under different documented processes. That is not a claim that the products can never be compared in a controlled experiment; only that these two capacity labels, as published, do not constitute one.

Throughput is not silver loading

Ilford’s same July 2010 sheet separately states that silver in a film fixing bath can rise to 8–10 g/l without serious effect. That sentence sits in the silver-concentration section. The capacity table’s 24-roll figure sits elsewhere. The sheet does not say that twenty-four rolls produce 8–10 g/l, or that the capacity number was derived from that silver band. No silver-per-roll figure is calculated here, because the document does not supply one. What fixer exhaustion actually means separates those throughput and permanence thresholds on the same manufacturer sheets. Kodak’s pack-in publishes the same 120-roll figure with a T-MAX exhaustion warning but no T-MAX capacity cut; see T-MAX exhausts fixer faster — no capacity cut in these Kodak sheets. Kodak’s unlabeled Solution B on that pack-in is why bath composition cannot be assumed identical for ranking; see Kodak names Rapid Fixer Solution B a hardener on E-103CF — not on the pack-in.

Kodak’s pack-in examined here publishes no silver-concentration threshold at all. That is a statement about this instruction sheet, not a claim that no Kodak document anywhere ever published one.

So the cross-brand efficiency reading fails twice. First, the capacity labels are not defined on matched processes. Second, even inside Ilford’s sheet, roll throughput and silver tolerance are not explicitly linked, and Kodak’s sheet does not supply a silver figure to compare. Without that bridge, a higher rolls-per-litre label is not evidence of lower silver loading per roll, longer useful life under a permanence criterion, or a better product.

What the ratio can and cannot support

The exact conversion supports one narrow claim: Kodak’s published 120 rolls per US gallon is approximately 31.7 rolls per litre on the exact US-gallon basis, and that converted figure is larger than Ilford’s published 24 rolls per litre by a factor of about 1.32.

It does not support a product-efficiency ranking from the labels alone. The process matrix above is the information gain: each capacity attaches to a dated sheet, a working-bath description, a dilution or preparation framing, a temperature/time band, and a replenishment figure, and the silver-loading bridge between the two throughput numbers is missing.

References